- Current or previous cigarette smoker with a history of cigarette smoking of ≥ 10 pack-years: number of pack years = (number of cigarettes per day/20) × number of years smoked) (e.g., 10 pack-years is equal to 20 cigarettes per day for 10 years, or 10 cigarettes per day for 20 years). Previous smokers are defined as those who stopped smoking at least 3 months prior to the study visit.